I have a 2002 F150 XLT with manual air. When the control is in the vent position, it blows air through the vent, then cycles through the defroster, and so forth. When you set control to heater, it blows through the defroster, but not through the heater. I have tried a different set of switches/temp control/fan control unit from a good running vehicle, but not differerence in operation. Help??

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